2009

We have analyzed parameters of 457 upwardinitiated negative lightning flashes to the Gaisberg tower (GBT) in Austria recorded from 2000 to 2007. Lightning to the radio tower of a height of 100 m and located on a mountain 1287 m above sea level occurs almost independent of season, although a pronounced thunderstorm season exists in Austria during the summer months. A total of 30% of the upward-initiated negative flashes exhibited one or more return strokes with an average multiplicity of 4.4 and a geometric mean interstroke interval of 17.3 ms. The remaining 70% of upward-initiated negative flashes showed an initial continuous current (ICC) only, either with (22%) or without (48%) superimposed current pulses of peak currents greater than 2 kA. Median of total transferred charge to ground is about 30% lower in summer (26.8 C) than in other seasons (39.0 C). For the return stroke current pulses, we determined a median peak current of 9.2 kA (σlog10 = 0.25) and a median pulse charge of 0.51 C (σlog10 = 0.39). For current pulses superimposed on the ICC, we determined a median of 4.2 kA (σlog10 = 0.26) with 22 kA peak current measured for the largest ICC pulse.

In this paper, we review and evaluate the definitions and methods that could be used to estimate the effective height of a given tower on mountaintop based on the statistical observations. We derive the effective heights based on Rizk’s lightning attachment model, which are less than those predicted by the earlier methods based on statistical observations. Then we perform sensitivity analysis to evaluate the effect of uncertainties in model parameters that influence the effective height. Variations in the effective height as a function of model parameters, including the final quasi-stationary leader gradient, minimum positive streamer gradient, upward connecting positive leader speed, and mountain base radius, are presented, with Gaisberg tower as the example. It’s found that the effective height depends primarily on the structure height, mountain shape and upward positive leader speed. This new approach presented here can be employed to estimate the effective height for towers for which no lightning incidence data needed for the earlier methods are available. And the information could be also used in designing lightning protection of communication/transmission line towers and masts on mountain tops.

Benford’s law applies to a wide variety of natural and man-made data sets and it has been successfully applied to test the integrity of data and to detect possible fraud and anomalous results in economy, politics and management studies. In this paper, we investigate the applicability of Benford’s law to lightning data. To do this, we used lightning data in Switzerland obtained using the European Cooperation of Lightning Detection (EUCLID) network. The considered data set consists of the total number of negative cloud-to-ground (CG) flashes per day in Switzerland for the period from 1999 till 2007. It is shown that the obtained distribution is in very good agreement with Benford’s law. The same analysis was repeated considering in the data set only the flashes containing return strokes with absolute peak currents lower than 2 kA, for which the detection efficiency of the lightning location network is expected to be lower. The resulting distribution shows less agreement with the Benford’s law, especially for the first, third and eighth digits, for which significant differences are obtained. The obtained results suggest that Benford’s law may find an interesting application in the evaluation of detection efficiency of a given lightning location network.

Monitoring units for the measurement of high-frequency voltage transients have been in operation at three different busses of an Italian medium voltage (MV) distribution feeder, mainly composed by overhead lines, in March 2007 – August 2008. The feeder is located in a region characterized by a high ground flash density value (4 flashes/km2/yr); many of the recorded voltage transients may be correlated with the lightning events detected for the same region by the Lightning Location System (LLS) CESISIRF. The paper presents some experimental results obtained using the monitoring units and their comparison with computer results obtained using a LIOV-EMTP model of the considered MV feeder. A procedure aimed at achieving the best fit between measurements and calculations, which takes into account the uncertainties associated with LLS data, is also presented.

In this paper we show the first data of time correlated lightning video and electric field measurements in Austria. During one thunderstorm on the June 1st, 2008 we recorded nine positive single stroke flashes to ground. The Austrian lightning location system (LLS) detected all nine positive flashes, although one was categorized as intracloud discharge. Additionally to the nine cloud-to-ground flashes the LLS detected some intracloud discharges and misclassified them as cloud-to-ground discharges. We further show some indication that positive flashes in Austria can also exhibit high peak currents and long continuing currents as shown for Brazil [Saba et al., 2006].

CIGRE TF C4.404 has recently submitted a comprehensive report for publication, dealing with the effects of performance characteristics of lightning location systems (LLS) on lightning parameters based on data from such systems. This paper will provide an overview and summary of this extensive report. Lightning parameters are essential input variables to procedures for estimating the lightning performance of transmission lines. Parameters that are typically derived from LLS observations are the ground flash density (GFD), ground stroke density (GSD), peak current distribution, flash multiplicity, and polarity. LLS upgrades and/or LLS expansions are causing changes in the network performance that result in changes in LLS-inferred lightning parameters. The CIGRE report discusses the effect of using different location methods in terms of required number of sensors to obtain a location. For example, median peak current (absolute value) increased by 47%, from -9.8 kA to -14.4 kA, when data from combined direction-finding and time-of-arrival sensors were reprocessed using only the time information and requiring 4 sensors to compute a location. This effect is reduced with shorter sensor baseline distances, or (equivalently) with greater sensor sensitivity. Direct measurements of currents in lightning striking instrumented towers or in triggered lightning allow estimation of all three major performance characteristics of LLS’s - detection efficiency (DE), for strokes and flashes, location accuracy, and peak current estimation errors. By deploying most recent technology of sensors a flash DE of 95% or higher is achievable. In a network with small sensor baselines and low sensor threshold a flash DE close to 100% is possible. Corresponding stroke DE is generally lower, but can reach values in the range of 80-90%. Peak current estimates given by LLS in the United States (NLDN) and Austria (ALDIS) are on average in reasonable agreement with the directly measured peak currents in triggered lightning and at electrically short towers, respectively, although significant differences (up to 50%) are observed for individual strokes, likely caused by the natural variation in return stroke speed.

This study describes the environmental atmospheric characteristics in the vicinity of different types of severe convective storms in Europe during the warm seasons in 2006 and 2007. 3406 severe weather events from the European SevereWeather Database ESWD were investigated to get information about different types of severe local storms, such as significant or weak tornadoes, large hail, damaging winds, and heavy precipitation. These data were combined with EUCLID (European Cooperation for Lightning Detection) lightning data to distinguish and classify thunderstorm activity on a European scale into seven categories: none, weak and 5 types of severe thunderstorms. Sounding parameters in close proximity to reported events were derived from daily high-resolution T799 ECMWF (European Centre for Medium-range Weather Forecasts) analyses. We found from the sounding-derived parameters in Europe: 1) Instability indices and CAPE have considerable skill to predict the occurrence of thunderstorms and the probability of severe events. 2) Low level moisture can be used as a predictor to distinguish between significant tornadoes or non-severe convection. 3) Most of the events associated with wind gusts during strong synoptic flow situations reveal the downward transport of momentum as a very important factor. 4) While deep-layer shear discriminates well between severe and non-severe events, the storm-relative helicity in the 0–1 km and especially in the 0–3 km layer adjacent to the ground has more skill in distinguishing between environments favouring significant tornadoes and wind gusts versus other severe events. Additionally, composite parameters that combine measurements of buoyancy, vertical shear and low level moisture have been tested to discriminate between severe events.